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Huygen's construction or Huygen's principle

Each point on a primary wavefront serves as the source of spherical secondary wavelets that advance at the wavespeed for the propagating medium. The primary wavefront at some later time is the envelope of these wavelets.

Law of Malus

Polarizing angle

Scattering

Birefringence

complicated phenomenon that occurs in calcite and other noncubic crystals and in some stressed plastics, such as cellophane. In such cases, light is separated into two rays: the ordinary ray and the extraordinary ray.

Phosphorescent materials

Stimulated emission

when the atom is initially in an excited state of energy EH and the energy of the incident photon is EH-EL where EH and EL are the energies of higher and lower energy states. The oscillating electromagnetic field associated with the incident photon can stimulate the excited atom, which then emits a photon in the same direction as the incident photon and in phase with it.

Focal ray

Radial ray

Sign conventions for reflection

s is positive when the object is on the incident-light side of the mirror. s' is positive if the image is on the reflected-light side of the mirror. r, and thus f, is positive if the mirror is concave so the center of curvature is on the reflected-light side of the mirror.

Lateral magnification

Refraction at a single surface

Sign conventions for refraction

s is positive for objects on the incident-light side of the surface. s' is positive for images on the refracted-light side of the surface. r is positive if the center of curvature is on the refracted-light side.